1. Field of the Invention
The present invention principally relates to fuel injected engines. More particularly, the present invention relates to a control strategy for controlling fuel injectors at the time of startup.
2. Description of the Related Art
In all fields of engine design, there is an increasing emphasis on obtaining more effective emission control, better fuel economy and, at the same time, continued high or higher power output. This trend has resulted in the substitution of fuel injection systems for carburetors as the engine charge former.
Fuel injection systems typically inject fuel into the air intake manifold. However, in order to obtain still better engine performance, direct injection systems are being considered. Direct fuel injection systems inject fuel directly into the combustion chamber. These systems potentially have significant advantages over typical fuel injection systems such as improved emission control.
In a direct injection system, a fuel injector is typically positioned in a cavity that is defined by a cylinder head. The nozzle of the fuel injector is exposed to the combustion chamber through an opening extending from the cavity so that the fuel may be injected directly into the combustion chamber.
In a manifold injection system, a fuel injector usually is disposed at a point along the induction system, downstream of a throttle device (e.g., a throttle valve). In many applications, the fuel injector is mounted at a location in close proximity to the combustion chamber, such as, for example, next to the intake port in a four-cycle engine. In this manner, fuel is injected into the air charge just before entering the combustion chamber.
The fuel injector, which is used with both direct and manifold injection systems, typically includes a needle valve that is actuated by an electromagnetic solenoid. The needle valve closes the nozzle of the fuel injector when the solenoid is deenergized. The needle valves mates with a valve seat to prevent passage of fuel across the valve. When the solenoid is energized, the needle valve moves away from the valve seat to form a clearance. Pressurized fuel is injected through this clearance, which typically is on the order of several-ten to several-hundred microns (i.e., micrometers).
In direction injection systems, as well as in some manifold injection configurations, the injector nozzle often is exposed to extremely high temperature which under some operating conditions can affect injector performance. For instance, a certain amount of liquid fuel, which contains heavy oil components, typically exists on the injector nozzle immediately after injection. If the heat in the injector nozzle exceeds the distillation temperature of the liquid fuel (for example, 90% of gasoline components evaporate at around 150.degree.), the valve seat tends to dry and heavy oil components deposit on the valve seat and/or injector nozzle. Excessive deposits of the heavy oil components can gum the valve seat and needle valve. Such deposits can accumulate under some extreme operation conditions (e.g., extended running periods during hot weather) to a degree that causes the needle valve to stick with the valve seat. The resulting bond between the valve seat and the needle valve prevents the fuel injector, and hence the engine, from functioning properly, if at all.
This phenomenon is exacerbated with engines employed in outboard motors. An outboard motor engine commonly is disposed with its crankshaft in a vertical orientation. The cylinders and fuel injectors of the engine consequently assume a generally horizontal position. This arrangement of the fuel injectors often traps air within the fuel injectors, and consequently, the valve seat become dry more frequently. Increased deposits of the heavy oil components of the fuel thus occurs on the valve seats of fuel injectors in an outboard motor. Fuel injector stiction thus occurs more often with outboard motor engines than with engines used in other applications (e.g., with automobile engines).